One final lesson—a dying man's last letter to his only son that completes the young man's education. My dear son:— The doctors have left and I am told that in a few hours I shall die. In my lifetime the world has progressed from the chaotic turmoil of the early Atomic era to the peacefulness and tranquility of our present age, and I die content. For ten years I have instructed you in all that you will need for the future. One final lesson remains to be taught. On the wall of my bedchamber hangs